Leviim Were the Smallest

כָּל־פְּקוּדֵי הַלְּוִיִם אֲשֶׁר פָּקַד מֹשֶׁה וְּאַהֲרֹן עַל־פִי יְּהוָּה לְּמִשְּפְּחֹתָּם כָּל־זָּכָּר מִבֶׁן־חֹדֶׁש וָּמַעְּלָּה שְּנַיִם וְּעֶׁשְּרִ ים אָּלֶׁף :
BeMidbar 3, 39: The total head-count of male Leviim whose family (lineage was verified) from the age of one month and upward, counted by Moshe and Aharon by G-d’s word was 22,000.

The count of the Leviim was from the age of a month and older. The rest of tribes were counted from the age of twenty. Yet, the Leviim had the smallest population of all the tribes.

Census from Parshas BeMidbar

• Yehudah 74,600
• Dan 64,700
• Shimeon 59,300
• Zevulun 57,400
• Yissachar 54,400
• Naftoli 53,400
• Reuven 46,500
• Gad 45,650
• Asher 41,500
• Efraim 40,500
• Benyamin 35,400
• Manasheh 32,200
• Leviim 22,000

Ten explanations for this phenomenon are offered:

1. Some of the Leviim had the responsibility to carry the holiest of the Mishkan vessels. If they were not treated with proper respect it would bring about their death. (Rashi on Bereshis 29,34) Some raise a question on this explanation, as the counting in our Parsha is before the tasks were allotted. However, we can say that because the future held this awesome responsibility, it was an indication that the Leviim were on an elevated level, closer to HaShem than any other tribe. Therefore, any other shortcoming in their spiritual responsibilities were also dealt with harshly, as is derived from Tehillim 50,3, "usvivov nisaroh m'ode."
2. The population explosion of the Bnei Yisrael in Egypt was nothing short of miraculous. The Torah says, "kaa'sher y'anu oso kein yirbeh," - as they pained the Bnei Yisrael, so did they multiply. The increase in the number of Bnei Yisrael was proportionate to their suffering. Since the tribe of Levi was not subjugated to slavery, as per M.R. Shmos 5,16, they did not multiply in a miraculous manner. (Ramban)
3. Our patriarch Yaakov expressed himself with negative terms towards Shimon and Levi (Bereshis 49:5,6,7). This caused a limitation in their growth. Shimon's limitation takes place at the later counting (Bemidbar 26,14). (Ramban)
4. The Leviim were the spiritual leaders of the Bnei Yisrael from the earliest days in Egypt. Their total immersion in Torah weakened them physically, "Torah ma'teshes kocho shel odom" (Gemara Sanhedrin 26b). Because they were physically weak they reproduced in a limited manner. (Tzror Hamor)
5. The need for the Bnei Yisrael to be a large population was so that when they would enter Eretz Yisroel they would occupy a large area so that they would not be overpowered by wild animals, "pen tirbeh o'lecho chayas hasadeh" (Devarim 7,22). Since the Leviim would not inherit a large parcel of land and in the main would reside close to the Beis HaMikdash, there was no need to have a large number of Leviim. (Abarbanel)